Fuel gases, such as natural gas and hydrogen gas, are promising alternatives to the traditional petroleum-based energy sources consumed by automotive vehicles. They are generally cleaner burning than traditional petroleum-based gasoline and diesel fuels and, thus, are better for the environment. One challenge encountered with the use of fuel gases, however, is how to store a sufficient amount of fuel gas aboard an automotive vehicle so that reasonable driving distances can be achieved between fill-ups. To this end, two storage approaches are typically employed when attempting to satisfy mobile on-board vehicle fuel gas storage needs: storing fuel gas in a compressed state or a solid state.
Natural gas, for instance, can be stored in a compressed state (compressed natural gas or CNG) or an adsorbed state (adsorbed natural gas or ANG). Compressed natural gas is stored at high pressures to less than 1% of the volume it would normally occupy at standard temperature and pressure (STP). Tank pressures of 150 bar to 250 bar are typically needed to achieve this level of compression. Adsorbed natural gas relies on a natural gas storage material that can adsorb and store natural gas in a solid state at an energy density comparable to compressed natural gas but at a much lower tank pressure of 60 bar or less. In a similar vein, hydrogen gas can be stored in a compressed state or by chemical uptake at an energy density comparable to compressed hydrogen gas but at a much lower tank pressure. Storing hydrogen gas by chemical uptake involves reversibly charging hydrogen gas on a storage material such as a metal hydride or a complex metal hydride like various alanates and amides. The solid state storage of natural gas and hydrogen gas on an ANG storage material and a hydrogen storage material, respectively, share similar thermodynamics in that the adsorption/chemical uptake processes are exothermic in nature while their eventual release is endothermic.
A design consideration that factors into the commercial demand and viability of on-board fuel gas storage tanks that utilize a gas storage material—and all vehicle fuel tanks for that matter—is “conformability.” The concept of tank conformability relates to the flexibility of the tank structure and how easily it can be adapted to fit the available packing requirements across many different vehicle platforms. The fuel gas storage tanks employed to date—for both compressed and solid state fuel gas storage—have largely been shaped as cylinders or spheres and are oftentimes made of thick and/or heavy materials. These tank constructions have been used to resist the forces exerted by the associated pressures from inside the tanks. But cylindrically- and spherically-shaped storage tanks are generally considered to be quite non-conformable since they do not always satisfy packaging requirements demanded in automotive vehicles and/or they are unable to fully utilize the space designated for the tank on a vehicle platform. The use of thick and/or heavy materials to fabricate the tank can also make the tank too heavy for some vehicle applications.